This started as a tradition. George Washington set the pattern by choosing to only serving for two terms. All of the presidents that followed him followed his example and didn't serve more than two full terms. That is until Franklin D. Roosevelt. Roosevelt was elected elected in 1932 on a promise to pull the Country out of the Great Depression and stayed on to lead this nation through most of World War II. In retrospect, Roosevelt probably should not have run for his fourth term in 1944. The fact that his health was failing at this time was concealed from the public and it is thought by many that his failing health was also affecting his judgement. There was also the fact that many people began to think that there was a sense of ownership about his administration. That is many thought that Roosevelt felt like he owned the presidency and that he was loosing touch with the people of the country. (This was an attitude that was most prominent among Republicans.) So, in 1951 the twenty-second amendment to the Constitution was ratified. This amendment limits a person to only being able to be elected to the office of President twice (only once if the president becomes president by replacing the previous president because he left office less than halfway through his term). So, what was once tradition has now been constitutionally mandated. In another answer around here, an interesting loophole in this amendment was pointed out. There is nothing in this amendment to stop a person who had served his two terms as President from then becoming Vice President. If such a situation were to occur and the then President were to prematurely leave office, then the previous President would be able to serve a third term without being elected to it.